This invention relates generally to the field of large aperture optical systems. More particularly, this invention relates to extendible large aperture mirror systems having a phased array of mirror segments and a method for controlling the same.
Large aperture optical systems are currently in use for remote sensing, communications and radiation beam transport. In addition to being operated in a receiving mode, a large telescope may also be used as a transmitting element, or beam director. In some applications, such as astronomy, large telescopes are used to make high resolution images of extremely faint objects. Such large telescopes are not restricted to just visible wavelength operation. On the ground, large diameter telescopes have been built for radio, infrared, and optical wavelengths, and in space telescopes are also used in the ultraviolet and x-ray regions of the electromagnetic spectrum.
At radio wavelengths, cassegrain antennas with apertures of up to 100 meters have been built. The primary mirror of a large radio telescope is always constructed of a multitude of panels or mirror segments. Each segment is manufactured to the correct shape to high accuracy before it is installed on the support. In the past, surveying instruments were used to accurately locate the reflecting panels typically while the antenna is pointed at the zenith. However, when the antenna is moved, the supporting structure deforms and disturbs the panel positions, thereby degrading the reflector accuracy and decreasing the antenna gain. These deformations lead to distorted wavefronts and degraded performance.
In view of the foregoing, there is a need for developing methods and apparatus by which the segments in a large mirror may be controlled to correct and compensate for optically distorted wavefronts. One such method and apparatus is disclosed in U.S. Pat. No. 4,825,062 which is assigned to the assignee hereof and fully incorporated herein by reference. U.S. Pat. No. 4,825,062 describes a mirror system which is also known by the acronym "PAMELA" and which stands for Phased Array Mirror Extendible Large Aperture. In PAMELA, subaperture tilt information is derived by sensing an incoming optical wavefront. That wavefront is then used as part of an adaptive optical system to compensate for aberrations both in the optical system itself and those due to external effects such as atmospheric turbulence.
The PAMELA system includes a plurality of optical mirror segments each having opposed front and rear surfaces. These surfaces are bounded by a plurality of side surfaces. The segments cooperatively receive and reflect the electromagnetic beam at the front surface of each segment. A plurality of displacement sensors generates signals indicative of the relative position of the front surface of the segments. A plurality of displacement actuators, each responsive to control signals, are connected to the rear surfaces of the segments. That is, each rear surface is connected at a corresponding number of points to displacement actuators which generate linear and angular segment movement relative to a segment axis substantially perpendicular to the segment front and rear surfaces that substantially correspond to a mean segment position approximating the desired ideal optical figure. An optical wavefront sensor receives a portion of an electromagnetic reference beam and provides signals indicative of its wavefront distortion relative to an ideal wavefront. A controller transmits these signals directly to each segment, wherein in-situ actuator control signals are generated to position the mirror segments so as to generate a conjugate phase reflected electromagnetic beam without the need for external wavefront reconstruction addressing the full plurality of mirror segments on a one-to-one basis.
While well suited for its intended function, the PAMELA system disclosed in U.S. Pat. No. 4,825,062 is not entirely satisfactory in its control of the typically thousands of mirror segments defining large aperture phased array mirror systems. Accordingly, there continues to be a need for improved methods and apparatus for precisely controlling the multiplicity of mirror segments in a large mirror in order to maintain an accurate overall shape in the absence of an optical reference beam.